don't think I have invested that much into 3E, but I notice that even when playing 3E, a lot of the books I had went mostly unused. Too many stuff was to specific, to out there, or too weak... And I liked to homebrew a lot, too.
I jumped on the 4E bandwagon pretty early, and the little material I was able to get so far gives me the impression I always get something that I at least consider using at some point...
Well, time will tell if this will hold true over a longer period of time and more books...
Mustrum Ridcully
When 4e Eberron comes out I will buy those if only to take stuff out to translate to 3.5. NPCs and the whole who's who, what's what and where's where.
By reading the Siberys Seven SH and its glossary you can see how I borrow stuff regularly